Conventionally, as shown in FIG. 4 and FIG. 5, a stratified scavenging two-cycle engine, having a fluid mixture flow passage (not shown) for supplying a fluid mixture is connected to a crankcase 1 and an air flow passage 2 for supplying air is connected to a scavenging flow passage 3, is known. A check-valve 20 is provided at the outlet port of the air flow passage 2. The check-valve 20 is composed of a reed valve and is constructed to permit a flow from the air flow passage 2 to the scavenging flow passage 3 and prevent a flow from the scavenging flow passage 3 to the air flow passage 2.
Meanwhile, the scavenging flow passage 3 extends between the crankcase 1 and a cylinder block 4 so as to allow communication between a crank chamber la and a cylinder chamber 4a. A scavenging port 3a leading to the scavenging flow passage 3 is opened to a cylinder inner surface 4b, and an exhaust port (not shown) is opened thereto for exhausting combustion gas.
Further, the crankcase 1 is provided with a crankshaft 5, and a piston 7 is coupled to the crankshaft 5 with a connecting rod 6 between them. The piston 7 is fitted in the cylinder inner surface 4b and freely moves along an axial direction of the cylinder inner surface 4b. Further, the cylinder block 4 is provided with a cylinder head 8, which is provided with an ignition plug 9.
In the stratified scavenging two-cycle engine configured as above, as the piston 7 ascends, the pressure inside the crank chamber 1a starts to drop, and the scavenging port 3a and the exhaust port are sequentially closed. As a result, the fluid mixture in the cylinder chamber 4a is compressed, and the fluid mixture supplied from the fluid mixture flow passage is passed into the crank chamber 1a. In this situation, air also enters the crank chamber 1a through the scavenging flow passage 3 from the air flow passage 2.
When the piston 7 reaches an area in the vicinity of an upper-most position, the fluid mixture in the cylinder chamber 4a is ignited by means of the ignition plug 9, and thereby the pressure inside the cylinder chamber 4a rises and the piston 7 descends. When the piston 7 descends to a predetermined position, the exhaust port and the scavenging port 3a are sequentially opened. As a result of the exhaust port being opened, combustion gas is exhausted from the exhaust port, thereby the pressure inside the cylinder chamber 4a abruptly drops. As a result of the scavenging port 3a being opened, the air accumulated in the scavenging flow passage 3 spurts into the cylinder chamber 4a from the scavenging port 3a, and the combustion gas staying in the cylinder chamber 4a is compulsorily discharged from the exhaust port by the air. Thereafter, the fluid mixture in the crank chamber 1a enters the cylinder chamber 4a through the scavenging flow passage 3 from the scavenging port 3a. Thus the scavenging operation is completed.
Again the piston 7 ascends, and the aforesaid cycle is repeated once more.
According to the stratified scavenging two-cycle engine configured as above, the inside of the cylinder chamber 4a is scavenged first by air, and combustible gas is prevented from being discharged as a result of the fluid mixture blowing through, therefore obtaining a disadvantage that the exhaust gas is uncleaned.
In the aforesaid stratified scavenging two-cycle engine, a portion of the fluid mixture stays inside the scavenging flow passage 3 at a point in time when scavenging is completed, but most of the remaining fluid mixture is forced out toward the crank chamber 1a by the air supplied from the air flow passage 2 and is replaced by fresh air. The fluid mixture remaining at the exhaust port 3a of the scavenging flow passage 3, however, cannot be forced out toward the crank chamber 1a and stays there as is. As a result, at the time of starting scavenging, fluid mixture remaining at the scavenging port 3a enters the cylinder chamber 4a, and the fluid mixture blows out of the exhaust port, thereby causing the disadvantage in the form of unclean exhaust gas.